Bond-orbital theory of linear and nonlinear electronic response in ionic crystals. II. Nonlinear response

A bond-orbital theory of linear electronic response set out in the preceding companion paper (I) is expanded to include nonlinear response. Attention is focused on the third-order susceptibility ${\chi }_{\mathrm{ijkl}}^{\mathrm{}\left(3\right)\mathrm{}}$ (particularly as measured by the nonlinear refractive index $n_2$) in pretransition-metal halides and chalcogenides. Root-mean-square accuracy over 11 halides for relative $n_2$ values is about 9%, and points to an essential absence of local-field effects in bond-orbital response at least for halides. The latter conclusion enables us to make an absolute calibration for $n_2$—a question of some considerable controversy among different experimental groups.